There is a growing demand in the mobile communication area for a system having the ability to download large blocks of data to a Mobile Station (MS) on demand at a reasonable rate. Such data could for example be web pages from the Internet, possibly including video clips or similar. Typically a particular MS will only require such data intermittently, so fixed bandwidth dedicated links are not appropriate. To meet this equipment in UMTS, a High-Speed Downlink Packet Access (HSDPA) scheme is being developed which may facilitate transfer of packet data to a mobile station at up to 4 Mbps.
A conventional component of a packet data transmission system is an ARQ (Automatic Repeat request) process, for handling data packets received in error. For example, consider downlink packet transmission from a Base Station (BS) to a Mobile Station (MS) in HSDPA. When the MS receives a data packet it determines whether the packet has been corrupted, for example using Cyclic Redundancy Check (CRC) information. It then transmits a signal in a field allocated for this purpose to the BS, with a first signal used as an acknowledgement (ACK), to indicate that the packet was successfully received, and a second signal used as a negative acknowledgement (NACK), to indicate that the packet was received but corrupted. The signals may for example be different codewords or the same codeword transmitted at different powers. The base station (BS) requires an appropriate position for a decision threshold to be set so that it can decode the ACK/NACK messages correctly.
Since packet transmission is typically intermittent, discontinuous transmission (DTX) may typically be employed, so that nothing is transmitted by the mobile station (MS) in the ACK/NACK field unless a data packet has been received. In a typical scenario, the probability of the mobile station (MS) failing to detect a data packet that has been sent might be 1%. In this case it is desirable for the base station (BS) to interpret the DTX as if it were a NACK, so that the packet may be retransmitted to the MS. Interpreting a discontinuous transmission (DTX) as a NACK may be achieved either by offsetting the decision threshold at the BS towards the ACK signal, or by means of the mobile station (MS) transmitting a NACK in every ACK/NACK field which does not correspond to a packet with correct cyclic redundancy check (CRC), whether or not a packet was detected.
A problem with the mobile station (MS) transmitting in every ACK/NACK field is that uplink interference is significantly increased, and in addition the mobile station (MS) battery life is reduced. This is a particular problem when the packet traffic is bursty (as is often the case), resulting in the mobile station (MS) being required to transmit in many ACK/NACK fields when no packet had been transmitted to it.
A problem with offsetting the threshold for deciding between ACK and NACK commands is that the transmit power of the ACK command needs to be increased, as discussed below, in order to achieve an acceptably low probability of an ACK being interpreted as a NACK. As the probability of the MS transmitting an ACK should be much greater than that of transmitting a NACK in a well-designed communication system, increasing the ACK transmit power will significantly increase the average transmit power required in the ACK/NACK field.
Consider a typical communication system which requires that the probability of misinterpreting an ACK as a NACK is less than 1% and the probability of misinterpreting a NACK as an ACK is less than 0.01%. Assuming that the probability of the mobile station (MS) not detecting a packet is 1%, then the probability of misinterpreting a discontinuous transmission (DTX) as a NACK should be less than 1% (so that the combined probability of the mobile station (MS) not receiving a packet and its DTX being interpreted as an ACK is the same as the probability of misinterpreting a NACK as an ACK, i.e. less than 0.01%). Simulations for typical mobile communication channels have shown that offsetting the decision threshold towards ACK enough to ensure that the probability of misinterpreting DTX as an ACK is less than 1% has the effect of requiring the ACK power to be greater than the NACK power, by as much as 10 or 20d8 in some scenarios.
One partial solution, disclosed in our co-pending United Kingdom patent application 0207696.6 (Applicant's reference PHGB 020034), is for the mobile station (MS) to transmit NACKs continuously after its initial ACK/NACK for as long as a timer is running. This avoids the need for the BS to offset its decision threshold, thereby reducing the required ACK power. However, a problem with this scheme is that the BS still has to offset its ACK/NACK decision threshold for the first of a series of packets, or alternatively tolerate a higher mis-detection probability for DTX after the first packet.